Lecture 22-24 Molecular Geometries and Covalent Bonding ... · Molecular Geometries and Covalent Bonding Theories . Molecular Geometries and Bonding Molecular Shapes ... more accurately

Molecular Geometries

and Bonding

Lecture 22-24 Molecular Geometries

and Covalent Bonding Theories


and Bonding

Molecular Shapes •  we’ve learned to draw Lewis structures and

account for all the valence electrons in a molecule.

•  But: Lewis structures are two dimensional and molecules are 3 dimensional objects.

•  The 3D structure is absolutely critical for understanding molecules.


and Bonding

Molecular Shapes •  geometry & shape of

molecule critical •  we can easily predict

the 3D structure of a molecule just by adding up:

•  bound atoms + lone pairs


and Bonding

What Determines the Shape of a Molecule?

•  atoms and lone pairs take up space and prefer to be as far from each other as possible

•  shape can be predicted from simple geometry

lone pair

bonds


and Bonding

“Things”

•  The central atom has four “things” around it. A “thing” is an atom or a lone pair of electrons.

•  # things = atoms plus lone pairs

•  Equivalent to bonding pairs and nonbonding pairs


and Bonding

Valence Shell Electron Pair Repulsion Theory (VSEPR)

“The best arrangement of a given number of things is the one that minimizes the repulsions among them.”


and Bonding

Arrangement These are the arrangement for two through six things around a central atom.

You must learn these!

number of things arrangement bond angles


and Bonding

Arrangments

•  All one must do is count the number of “things” in the Lewis structure.

•  The geometry will be that which corresponds to that number of “things.”


and Bonding

Molecular Arrangments

•  The geometry is often not the shape of the molecule, however.

•  The “shape” is defined by the positions of only the atoms in the molecules, not the lone pairs.


and Bonding

Arrangement vs. shape

Within each geometry, there might be more than one shape.


and Bonding

Linear arrangement two things

•  In this geometry, there is only one molecular geometry: linear.

•  NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the geometry is.

things geometry atoms lone pairs shape example


and Bonding

Trigonal Planar arrangement 3 things

•  There are two molecular geometries: –  Trigonal planar, if there are no lone pairs –  Bent, if there is a lone pair.

things geometry atoms lone pairs shape example


and Bonding

Lone pairs and Bond Angle

•  Lone pairs are physically larger than atoms.

•  Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.


and Bonding

Multiple Bonds and Bond Angles

•  Double and triple bonds place greater electron density on one side of the central atom than do single bonds.

•  Therefore, they also affect bond angles.


and Bonding

Tetrahedral arrangement 4 things

•  There are three molecular geometries: –  Tetrahedral, if no lone pairs –  Trigonal pyramidal if one is a lone pair –  Bent if there are two lone pairs

Things geometry atoms lone pairs shape example


and Bonding

Trigonal Bipyramidal arrangment 5 things

•  There are two distinct positions in this geometry: –  Axial –  Equatorial


and Bonding

Trigonal Bipyramidal arrangment

Lower-energy conformations result from having lone pairs in equatorial, rather than axial, positions in this geometry.


and Bonding

Trigonal Bipyramidal arrangement Things geometry atoms lone pairs shape example •  There are four

distinct molecular geometries in this domain: –  Trigonal

bipyramidal –  Seesaw –  T-shaped –  Linear


and Bonding

Octahedral arrangment 6 things

•  All positions are equivalent in the octahedral domain. •  There are three molecular geometries:

Things geometry atoms lone shape example pairs


and Bonding

Polarity

•  In Chapter 8 we discussed bond dipoles.

•  polar bonds versus polar molecules. We must think about the molecule as a whole.


and Bonding

Polarity

By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.


and Bonding

Polarity “The tractor pull”


and Bonding

Overlap and Bonding •  covalent bonds form when electrons are “shared.” •  But how, when the electrons are in these atomic

orbitals? Do atomic orbitals overlap? •  Yes.


and Bonding

Overlap and Bonding •  Increased overlap brings

the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion.

•  However, if atoms get too close, the internuclear repulsion greatly raises the energy.


and Bonding

Hybrid Orbitals

But how do you get tetrahedral, trigonal bipyramidal, and other geometries when the atomic orbitals seem to be at right angles from each other all the time?


and Bonding

Hybrid Orbitals

•  Consider beryllium: –  In its ground electronic

state, it would not be able to form bonds because it has no singly-occupied orbitals.


and Bonding

Hybrid Orbitals

But if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.


and Bonding

Hybrid Orbitals •  Mixing the s and p orbitals yields two degenerate

orbitals that are hybrids of the two orbitals. –  These sp hybrid orbitals have two lobes like a p orbital. –  One of the lobes is larger and more rounded as is the s

orbital.


and Bonding

Hybrid Orbitals •  These two degenerate orbitals would align

themselves 180° from each other. •  This is consistent with the observed geometry of

beryllium compounds: linear.


and Bonding

Hybrid Orbitals

•  With hybrid orbitals the orbital diagram for beryllium would look like this.

•  The sp orbitals are higher in energy than the 1s orbital but lower than the 2p.


and Bonding

Hybrid Orbitals

Using a similar model for boron leads to…


and Bonding

Hybrid Orbitals

…three degenerate sp2 orbitals.


and Bonding

Hybrid Orbitals

With carbon we get…


and Bonding

Hybrid Orbitals …four degenerate

sp3 orbitals.


and Bonding

Hybrid Orbitals

For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.


and Bonding

Hybrid Orbitals

This leads to five degenerate sp3d orbitals… …or six degenerate sp3d2 orbitals.


and Bonding

Hybrid Orbitals

Once you know the number of things around an atom, you know the hybridization state of the atom if you can count letters up to six.


and Bonding

Valence Bond Theory

•  Hybridization is a major player in this approach to bonding.

•  There are two ways orbitals can overlap to form bonds between atoms.


and Bonding

Sigma (σ) Bonds

•  Sigma bonds are characterized by –  Head-to-head overlap. –  Cylindrical symmetry of electron density about the

internuclear axis.


and Bonding

Pi (π) Bonds

•  Pi bonds are characterized by –  Side-to-side overlap. –  Electron density

above and below the internuclear axis.


and Bonding

Single Bonds

Single bonds are always σ bonds, because σ overlap is greater, resulting in a stronger bond and more energy lowering.


and Bonding

Multiple Bonds

In a multiple bond one of the bonds is a σ bond and the rest are π bonds.


and Bonding

Multiple Bonds •  Example:

formaldehyde an sp2 orbital on carbon overlaps in σ fashion with the corresponding orbital on the oxygen.

•  The unhybridized p orbitals overlap in π fashion.


and Bonding

Multiple Bonds

In triple bonds, as in acetylene, two sp orbitals form a σ bond between the carbons, and two pairs of p orbitals overlap in π fashion to form the two π bonds.


and Bonding

Delocalized Electrons: Resonance

When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion.

- - -

-¸ -¸

-¸

+ + +


and Bonding


•  each of the four atoms in the nitrate ion has a p orbital.

•  The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen.


and Bonding


This means the π electrons are not localized between the nitrogen and one of the oxygens, but rather are delocalized throughout the ion.


and Bonding

Resonance The organic molecule benzene has six σ bonds and a p orbital on each carbon atom.


and Bonding

Resonance

•  In reality the π electrons in benzene are not localized, but delocalized.

•  The even distribution of the π electrons in benzene makes the molecule unusually stable.


and Bonding

Valence bond theory

•  Hybridization to explain geometry.

•  What orbitals are involved in multiple bonds?

•  Delocalization and resonance.


and Bonding

Orbitals in Molecules •  Molecular orbital theory, another way to

look at bonding.

•  We will only, briefly look at diatomic molecules:


and Bonding

•  In MO theory, you combine atomic orbitals from each atom. For orbitals to combine: – Energy must be similar – Orbitals must overlap. – Orbitals must have the same symmetry

Orbitals in Molecules


and Bonding

•  What atomic orbitals can combine to form sigma (σ) bonds?


s + s

s + p

p + p


and Bonding

•  What atomic orbitals can combine to form pi (π) bonds?


p + p

p + d

Documents

Lecture 22-24 Molecular Geometries and Covalent Bonding ... · Molecular Geometries and Covalent Bonding Theories . Molecular Geometries and Bonding Molecular Shapes ... more accurately